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From "Feng Honghua (JIRA)" <j...@apache.org>
Subject [jira] [Created] (HBASE-10679) Both clients operating on a same region will get wrong scan results if the first scanner expires and the second scanner is created with the same scannerId
Date Wed, 05 Mar 2014 08:15:44 GMT
Feng Honghua created HBASE-10679:
------------------------------------

             Summary: Both clients operating on a same region will get wrong scan results
if the first scanner expires and the second scanner is created with the same scannerId
                 Key: HBASE-10679
                 URL: https://issues.apache.org/jira/browse/HBASE-10679
             Project: HBase
          Issue Type: Bug
          Components: regionserver
            Reporter: Feng Honghua
            Assignee: Feng Honghua
            Priority: Critical


The scenario is as below (both Client A and Client B scan against Region R)
# A opens a scanner SA on R, the scannerId is N, it successfully get its first row "a"
# SA's lease expires and it's removed from scanners
# B opens a scanner SB on R, the scannerId is N too. it successfully get its first row "m"
# A issues its second scan request with scannerId N, regionserver finds N is valid scannerId
and the region matches too. (since the region is always online on this regionserver and both
two scanners are against it), so it executes scan request on SB, returns "n" to A -- wrong!
(get data from other scanner, A expects row something like "b" that follows "a")
# B issues its second scan request with scannerId N, regionserver also thinks it's valid,
and executes scan on SB, return "o" to B -- wrong! (should return "n" but "n" has been scanned
out by A just now)

The consequence is both clients get wrong scan results:
# A gets data from scanner created by other client, its own scanner has expired and removed
# B misses data which should be gotten but has been wrongly scanned out by A

The root cause is scannerId generated by regionserver can't be guaranteed unique within regionserver's
whole lifecycle, *there is only guarantee that scannerIds of scanners that are currently still
valid (not expired) are unique*, so a same scannerId can present in scanners again after a
former scanner with this scannerId expires and has been removed from scanners. And if the
second scanner is against the same region, the bug arises.

Theoretically, the possibility of above scenario should be very rare(two consecutive scans
on a same region from two different clients get a same scannerId, and the first expires before
the second is created), but it does can happen, and once it happens, the consequence is severe(all
clients involved get wrong data), and should be extremely hard to diagnose/debug



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